The present invention relates to pressure transmitters for measuring pressures of process fluids in industrial processes. More specifically, the present invention relates to reducing adverse effects of hydrogen on performance of pressure transmitters.
Process pressure transmitters are used in a variety of applications to sense pressure (absolute, gage, or differential) within a process environment. Additionally, a process pressure transmitter can be used to sense differential pressure from two distinct points, such as at varying elevations along a tank and provide an indication of a fluid level within the tank. In some configurations, a thin, flexible isolation diaphragm and fill fluid separate the pressure sensitive element of the pressure transmitter from the process fluid. When process pressure is applied, the diaphragm of the pressure transmitter is displaced. This displacement is related to the process pressure and is converted electronically to an appropriate current, voltage, or digital output signal such as HART® (Highway Addressable Remote Transducer).
In order to ensure that the pressure sensed within the pressure transmitter is an accurate representation of the process pressure, it is important that the fill fluid be incompressible. While this may seem to be a relatively straight forward design criterion, it is known that certain types of oils will, over time, outgas or develop bubbles therein. Typically, these issues are addressed by selecting very high quality fill fluids; pre-processing the fill fluid to decrease the extent to which it may outgas over time; and through other relatively high cost techniques. Despite the various steps taken to reduce outgassing and the resulting effects, which increase the manufacturing costs of a pressure transmitter, adverse effects of gases in the fill fluid remain an issue in pressure transmitters.
One source of gas in pressure transmitter fill fluid is hydrogen atoms which permeate the isolation diaphragm. In all crystals, some of the lattice sites are unoccupied. These unoccupied sites are called vacancies. If one of the atoms on an adjacent site jumps into the vacancy, the atom is said to have diffused by a vacancy mechanism. This type of diffusion allows hydrogen atoms to permeate the isolation diaphragm. Because transmitter diaphragms are very thin, hydrogen atoms permeating through the diaphragm can unite to form molecular hydrogen. Because molecular hydrogen is too large to permeate back through the diaphragm, it gets trapped and forms bubbles in the fill fluid. These bubbles can severely affect transmitter performance.
In order to reduce the effects of hydrogen gas on pressure transmitter performance, care typically must be taken to prevent placing certain dissimilar metals in close proximity where atomic hydrogen could be generated. Positioning cadmium or cadmium-plated parts near high-nickel alloys, such as SST or Alloy C-276, in the presence of an electrolyte such as water, can result in the creation of a Ni Cad battery effect where atomic hydrogen is released. This atomic hydrogen can then permeate a thin diaphragm. In general, in applications where atomic hydrogen is present, materials that are not susceptible to permeation should be chosen. Metals that contain a lot of nickel are more susceptible to permeation. Increased temperatures also increase the rate of permeation.
Plating certain alloys common to pressure transmitters, such as Alloy-400, with gold provides protection against hydrogen permeation while providing the corrosion resistance of Alloy-400. However, with rising costs associated with gold, this technique for reducing hydrogen permeation can add significantly to the costs of manufacturing pressure transmitters.
As discussed above, in addition to hydrogen permeation, hydrogen can also form bubbles due to out-gassing from the SST casting. This can be a serious problem when module castings are not annealed. It also is a problem for high temperature, high vacuum applications. A common solution to out-gassing is to bake out the transmitter parts. This adds cost, but more importantly, the bake out time becomes a judgment. Hydrogen will out-gas forever so the bake out is ideally done only long enough such that any further out-gassing will not significantly affect performance. However, determining the proper bake out time can be difficult.
The discussion above is merely provided for general background information and is not intended to be used as an aid in determining the scope of any claimed subject matter.